1. Field of the Invention
The present invention relates to a duplex image transferring device for transferring toner images from a first and a second image carrier to both sides of a sheet or similar recording medium, and an image forming apparatus using the same.
2. Description of the Background Art
It is generally difficult to transfer two toner images formed by toner of the same polarity to both sides of a single sheet face to face. In light of this, a conventional duplex image transferring device is usually constructed to sequentially pass a sheet through an image transferring device and a fixing device, reverse the sheet, and again pass it through the same route, thereby forming toner images on both sides of the sheet. In such a switchback type of device, the toner image transferred to the sheet first by the first transfer is fixed on the sheet before the second transfer, which causes a reverse electric field to act on the toner image. This promotes desirable transfer of the two toner images to both sides of the sheet.
The switchback type of device, however, needs a sophisticated switchback mechanism for reversing the sheet and then returning it to the image transferring device. Further, switchback obstructs high-speed duplex image transfer. Moreover, the sheet carrying the toner image transferred by the first image transfer extends due to fixation and is therefore likely to dislocate the toner images formed on both sides thereof.
To solve the above problems, Japanese Patent Publication No. 51-13022 and Japanese Patent Laid-Open Publication Nos. 63-63057 and 2-259670, for example, each disclose a particular image forming apparatus using a pair of image carriers. Toner images formed by toner of opposite polarities are respectively formed on the pair of image carriers and then transferred to both sides of a sheet. This type of apparatus, however, need a pair of photoconductive drums, a pair of optical writing units, a pair of developing units and so forth different in specification from each other due to the different polarities of the toner images. The apparatus therefore requires a far greater number of parts than an apparatus of the type dealing with toner of the same polarity, making maintenance troublesome. Maintenance is further aggravated because the toner different only in chargeability from each other must be managed independently of each other.
To promote easy maintenance, there has been proposed an image forming apparatus of the type forming two toner images with toner of the same chargeability and charging, before the transfer of one toner image to a sheet, the toner image with a corona charger to the opposite polarity. This type of apparatus is taught in, e.g., Japanese Patent Laid-Open Publication Nos. 7-77851, 8-211664, 10-171264 and 10-97106 by way of example. Using toner of the same chargeability facilitates maintenance. Further, because the corona charger charges one toner image to the opposite polarity before transfer, two toner images of different polarities are electrostatically moved toward the sheet intervening between them. This makes it needless to transfer one toner image to the sheet beforehand and thereby implements duplex image transfer by a single pass.
However, even the apparatus described above has a problem that corona discharge for reversing the polarity of one toner image scatters the toner to a non-image area around the toner image.
As stated above, an image forming apparatus of the type reversing the polarity of one toner image with a corona charger brings about toner scattering although it solves the problems ascribable to switchback or the use of two different kinds of toner.
Image forming apparatuses in general use either one of dry toner and a developing liquid containing toner and a carrier liquid. We conducted a series of experiments with a test model of an image forming apparatus of the type using a developing liquid. The test model includes an image forming device, a sheet tray, a registration roller pair and so forth. The image forming device includes a photoconductive element or image carrier. Arranged around the drum are a corona charger, an optical writing unit, a developing device, an image transfer roller, discharging mans, and a drum cleaner. The test model forms a latent image on the drum with a conventional electrophotographic process. The developing device stores a developing liquid having viscosity of 100 cSt and containing 15 wt % of toner dispersed in silicone oil or similar insulative carrier liquid. The developing liquid is deposited on a developing roller. A power supply applies a bias for development to the developing roller, so that an electric field for development is formed at a developing position between the drum and the developing roller. The electric field causes the toner of the developing liquid to migrate toward the latent image formed on the drum by electrophoresis, thereby forming a corresponding toner image. The drum in rotation conveys the toner image to a nip between the drum and the image transfer roller.
A pickup roller pays out a sheet from the sheet tray in synchronism with the image formation of the image forming device. The registration roller pair nips the sheet and then drives it toward the nip at a preselected timing such that the leading edge of the sheet meets the leading edge of the toner image. A power supply applies a bias for image transfer to the image transfer roller, forming an electric field at the nip. The toner image is therefore transferred from the drum to the sheet due to the electric field and a nip pressure. After the image transfer, the drum cleaner cleans the surface of the drum with a cleaning blade.
In a machine for practical use, a fixing device is positioned at a preselected position, so that the sheet moved away from the nip is passed through the fixing device. The fixing device was intentionally removed from the test model for convenience.
One day, we conducted experiments with a certain intention by reusing, for a resource and cost saving purpose, sheets carrying unfixed images on one side thereof and transferring toner images to the other side of the same sheets. It was a surprise to find that toner images were transferred to the other side of each sheet without the unfixed toner image on one side of the same sheet being reversely transferred to the image transfer roller. We first doubted this result because the one-sided sheets had been simply stored over a long time after image transfer. However, the result of continuous transfer of toner images to both sides of sheets was the same as the above result. The experiments therefore taught us that duplex image transfer was achievable without resorting to two different kinds of toner or a corona charger for reversing the polarity of one toner image. Although some toner was left on the image transfer roller after duplex image transfer, it was negligible in practical use.
When coated sheets were substituted for plain sheets used for the experiments, the amount of toner left on the coated sheets was reduced to about one-half of the toner left on the plain sheets. When porous sheets, which are highly liquid-absorptive, were substituted for the plain sheets, the amount of residual toner was too great to be called xe2x80x9cresidual tonerxe2x80x9d and brought about reverse transfer. This was also true with OHP (OverHead Projector) sheets, which are not liquid-absorptive at all.
The results of the experiments described above suggest the following. When a second toner image is transferred to the other side of a sheet carrying a first toner image on one side (second transfer), the carrier liquid of the first toner image serves as a parting agent that causes the toner image to part from the image transfer roller and thereby obstructs reverse transfer. More specifically, the developing liquid or colored liquid contains far smaller toner grains than dry toner.
At the time of the first transfer, fine toner grains constituting a toner image densely gather at a sheet by electrophoresis under the action of an electrostatic force. The electrostatic force and nip pressure cooperate to press the toner grains against the sheet. As a result, the toner grains adhere more strongly to each other and form a single mass with hardly any carrier liquid intervening between the grains. In parallel with this, the sheet absorbs the carrier liquid of the developing liquid little by little. When the first transfer is about to end, the sheet absorbs most of the liquid carrier with only a small amount of carrier liquid remaining on the toner mass in the form of a layer.
At the time of the second transfer, a reverse electric field acts on the toner mass transferred to the sheet first. At this instant, the toner mass tends to rather bodily move in the reverse section than migrates by electrophoresis because the liquid carrier is short. The small amount of carrier liquid left on the toner mass intervenes between the toner mass and the image transfer roller and serves as a parting agent. As for a coated sheet lower in liquid absorbability than a plain sheet, a greater amount of liquid carrier remains than on a plain sheet. This suggests that the parting effect is further enhanced to obstruct reverse transfer more positively. As for a porous sheet highly liquid-absorptive, an amount of carrier liquid great enough to serve as a parting agent presumably does not remain on the toner mass, so that the toner mass is reversely transferred to the image transfer roller due to the electric field. Further, as for an OHP sheet not liquid-absorptive, an amount of carrier liquid great enough to serve as an electrophoresis medium rather than a parting agent remains on the toner mass, causing the toner to easily migrate in the reverse direction by electrophoresis under the action of the reverse electric field.
It is therefore an object of the present invention to provide a duplex image transferring device capable of transferring toner images to both sides of a recording medium without switching back a one-sided recording medium or using two kinds of toner different in chargeability or charging one toner image to the opposite polarity with a corona charger, and an image forming apparatus using the same.
A duplex image transferring method of the present invention begins with a step of bringing one side of a recording medium into contact with a first toner image, which is formed on a first image carrier by a colored liquid containing toner and a carrier liquid. A first electric field acts toward the recording medium in a forward direction to thereby transfer the first toner image to the one side of the recording medium. At the same time, a toner layer gathered at the recording medium and a carrier liquid layer left on the first image carrier are caused to part from each other. In a second step, the other side of the recording medium is brought into contact with a second toner image formed on a second image carrier by the color liquid. At this instant, first toner image, in which the liquid carrier layer contains the carrier liquid of an amount not great enough to serve as an electrophoresis medium for the toner layer, but sufficient to serve as a parting agent for the toner layer and a contact member contacting the toner layer, is maintained on the one side of the recording medium. Subsequently, a second electric field, which acts toward the recording medium and forward for the second toner image, but reverse for the first toner image, acts on the second toner image and first toner image to thereby transfer the second toner image to the other side of the recording medium.